Paper is manufactured in a continuous process starting with a papermaking stock consisting of fibers suspended in water. A web of fibers is formed by depositing fibers from the stock onto one or more forming wires in the forming section or wet end of a papermaking machine. The web leaves the former with a fiber content of ten to twenty percent and enters a pressing section where the fiber content is increased to thirty-five to fifty percent. From the pressing section the paper web enters a dryer section where the web is dried to ninety-five percent fiber and five percent moisture. Although paper making is a continue process, it must be started each time a new grade of paper is made or the web being formed breaks. Web breaks can be frequent, especially during initial setup of a new or modified papermaking machine.
The process of starting a papermaking machine is called threading the machine. As the web progresses through the former and the pressing section it is generally supported at all times by a forming wire or press felt. Thus the forming section and the press section generally thread automatically. Historically, threading the dryer section has been more problematic. In most existing papermaking machines two ropes are run side by side in a groove on the front face of the dryer section. The web is cut either at the former or in the press section to form a tail about four to twelve inches wide and the rest of the web is discarded and reprocessed into fiber. The tail is separated from the rest of the web at the end of the press section and the tail is blown onto the threading ropes which entangle the tail between the threading ropes and drag the tail through the dryers. When the tail successfully transits the dryer section the tail is gradually increased in width until the entire web passes through the dryer section and the threading operation is complete.
The use of ropes to effect threading of the dryer section has several disadvantages. First, as papermaking speeds have increased the reliability of the rope method of threading has decreased. In addition, the rope threading of the paper web offsets the tail to the outside of the dryers away from its normal path through the dryer section which can contribute to breakage of the tail as it is being threaded. At higher speeds the ropes can break and become entangled with components of the dryer section causing paper breaks and machine downtime. An improved approach to threading has been provided by using a damper within a reversing roll to selectively increase the vacuum drawn on that portion of the roll which contacts the tail during threading. Enclosed boxes have been inserted between two dryer rolls above the reversing roll, to allow a vacuum to be drawn between the box exterior walls and the moving felt. However, it would be desirable to more selectively apply the vacuum to the region of the tail during threading.
What is needed is an apparatus and method for threading the dryer section of a papermaking machine without the use of ropes.